The present invention relates, generally, to semiconductor devices and, more particularly, to thermal stresses affecting semiconductor devices.
Micro-Electrical-Mechanical-Systems (MEMS) such as sensors are widely used in applications such as automotive, household appliances, building ventilation, and in general industrial applications to sense a physical condition such as pressure, temperature, or acceleration, and to provide an electrical signal representative of the sensed physical condition. A conventional pressure sensor is constructed as a network of resistors in a resistive bridge configuration, wherein the resistive bridge has two terminals for coupling to power supply potentials and two terminals for providing a differential output signal. In pressure sensor applications, the bridge resistors are formed on a sensing diaphragm by implanting or diffusing impurity materials into a semiconductor substrate. When the sensor is exposed to the physical condition, one or more of the bridge resistors changes resistance, thereby unbalancing the resistive bridge and generating a differential output signal. As those skilled in the art are aware, the electrical signal is proportional to the imbalance of the resistor network.
A drawback of resistive bridge type sensors is that they produce a non-zero output electrical signal (i.e., offset voltage) at their output terminals with a null input applied. This non-zero offset electrical signal in response to the null input also varies from sensor to sensor due to process and manufacturing variations. It should be noted that a sensor is at a null state when the null input of the physical condition is applied, i.e., when the pressure on one side of the sensing diaphragm is equal to the pressure on an opposing side of the sensing diaphragm. Further, the output signal of these types of pressure sensors varies over the operating pressure range at a specified supply voltage. The change in the output signal over the operating pressure range also varies from sensor to sensor due to process and manufacturing variations. In addition, the offset parameter is sensitive to temperature in these types of sensors. Definitions of terms commonly used by those skilled in sensor art can be found in the second edition of the data book entitled "PRESSURE SENSOR DEVICE DATA" copyrighted in 1994 by Motorola, Inc.
Temperature coefficient of offset (Tco) is a measure of non-pressure induced stresses as a function of temperature that are placed on a semiconductor device such as a MEMS device and is expressed in microvolts per degree Celsius (.mu.V/.degree. C.). A large Tco makes it difficult to design over temperature. Thus, for ease of design using MEMS, inaccuracies, such as Tco, are preferably near zero. In practice, however, the Tco is typically not sufficiently near zero, thus requiring compensation to reduce the Tco to near zero.
One commonly used technique for calibrating and temperature compensating offset is to provide an offset resistor network that is laser trimmed. Drawbacks of this technique are that laser trimming each sensor is an irreversible process that is costly and increases the amount of time for manufacturing a sensor, i.e., increases cycle time.
While some techniques compensate for Tco, they are not directed at a specific cause. A specific cause of a high or undesirable, e.g. nonzero, Tco has not been identified. Accordingly, it would be advantageous to identify a cause or factor affecting the temperature coefficient of offset and to have a method and mechanism for adjusting that factor to decrease or adjust the Tco of a MEMS. It would be of further advantage for the Tco-adjusted MEMS to be manufacturable using common sensor manufacturing techniques.